Electronic products are physically decreasing in size at a rapid rate. At the same time, consumers have become more demanding in their expectations of the quality and reliability of the products they buy. In order to meet quality requirements, electronic product manufacturers must thoroughly test the product at various stages during the manufacturing process. However, as electronic devices become smaller, access to the critical portions (e.g., electrical circuit nodes) of the device necessary for testing the product has become more and more difficult. This problem is occurring in a time when many manufacturers are also facing an increasing need to test product components faster and more efficiently.
A printed circuit board (PCB) is subject to many different types of defects during the assembly process. Accordingly, various test and inspection techniques are employed to locate these defects. Today, there are three general test methods used to find PCB defects: electrical test, optical (or visual) inspection, and x-ray inspection. Of these, electrical test, and in particular a technique known as “in-circuit test”, is the most mature and most commonly used technique. However, as physical access to nodes on the PCB via bed-of-nails probing decreases, in-circuit test is becoming less effective.
One of the most prevalent defects on PCB assemblies today is missing devices. The devices are either never loaded onto the board or they fall off during the assembly process. Prior methods for detecting missing devices at the electrical test stage of the process include in-circuit test, functional test, capacitive measurement test, scan test, automated optical test, and automated x-ray test.
In-circuit test, including unpowered in-circuit analog test (for discrete analog components) and digital in-circuit test for digital components, utilizes an in-circuit tester. The in-circuit tester includes a bed-of-nails test-head having a number of tester interface pins. A fixture having a number of probes is mounted over the bed-of-nails of the tester such that the fixture probes align with and contact tester interface pins. A printed circuit board under test is mounted in the fixture such that the fixture probes electrically contact various nodes of interest on the PCB under test. Analog in-circuit tests detect missing components on the PCB under test by probing the appropriate nodes to which the component under test should be attached, and measuring the value, in appropriate units (e.g., resistance, capacitance, etc.), of the component under test. If the measured value is within predetermined limits of the expected value, the test infers that the component under test is indeed present.
Similarly, in functional test, input and output nodes on the board to which the component under test should be attached are probed, digital values are applied to the input nodes, and digital results are collected from the output nodes. If the correct results are collected, the test infers that the component under test is indeed present.
Capacitive measurement test, such as Agilent Technology's TestJet™ probe and technique (described in detail in U.S. Pat. No. 5,254,953 to Crook et al., and incorporated herein by reference for all that it teaches), detects when a device pin is not properly connected to its trace on the PCB. The technique uses an external plate, suspended over the device under test and separated from the lead frame by the plastic or ceramic material of the device housing The lead frame and external plate form a small capacitor that can be measured by stimulation with an AC source. When the device pin is not electrically connected to the trace, an additional capacitance results in series with the TestJet™ capacitor. This additional capacitance exists due to the tiny air gap between the pin and trace. This is a very small capacitance, much smaller than the TestJet™ capacitor, so the series combination of the TestJet™ and this additional pin capacitor is smaller than either capacitor. A threshold value can be set for each pin of each device under test to discriminate between present and absent devices.
Missing digital devices can often be detected using scan test methodologies based on IEEE 1149.1. However, scan test only works on devices that conform to the IEEE 1149.1 standard. Furthermore, even scan test requires some probing. Moreover, the absence of certain classes of devices connected in difficult topologies cannot be detected by electrical methods even if physical probing is provided. Parallel bypass capacitors are one example.
Another emerging technique for detecting missing devices on a PCB is through mechanical switching detection. In this technique, a spring-loaded probe attempts to probe a part where it should be located. If the part is present, the spring of the probe compresses to close a mechanical switch, which completes a circuit to allow current to flow. Thus, when the device is present, current is measurable in the circuit; likewise, when the device is not present, no current flows through the circuit. The mechanical switching detection technique is problematic in that it contains moving parts, making it susceptible to part failure, and requires physical contact of the component under test.
The above techniques each require at least some physical probing of the PCB nodes (with the exception of the mechanical switching technique) and are therefore ineffective for PCB assemblies with limited nodal access. To overcome loss of test coverage in non-probed areas of the PCB, alternate test methodologies have emerged. These include automated optical inspection (AOI) and automated x-ray inspection (AXI). Although these methodologies can detect missing devices very effectively, they each suffer from their own limitation and disadvantages. The major disadvantage of these techniques is that they require expensive manufacturing line equipment entirely separate from the in-circuit tester, and therefore also require an entirely new test step to be added to the manufacturing process. The cost of adding such machines to the manufacturing process may be appropriate in some cases, but in other cases the need to do so represents a large disadvantage to these methods.
Since most manufacturing lines already use electrical testers (primarily in-circuit testers), it would be beneficial to have the ability to detect missing devices during the in-circuit stage of the manufacturing process. However, due to decreased access to PCB nodes due to ever-decreasing node spacing, the current solutions for detecting missing devices on a PCB are becoming less viable. The primary reason for this is that most electrical techniques used today to check for missing components depend on physical access, especially for analog components.
Another force at work decreasing the viability of electrical testers in detecting missing PCB components is that some devices are electrically untestable even with probing. The primary example of this is parallel bypass capacitors. While it is theoretically possible (e.g., on the bench with a single device under test (DUT)) to detect a single missing capacitor, in practice such detection is often not possible. The tolerances and guardbands that must be added to the test limits completely hide small measurement differences due to a single (or even multiple) missing capacitors. As MSI and LSI are replaced by VLSI components, FPGAs and large ASICs, the ratio of bypass capacitors to digital components is increasing, which decreases the number of possible faults that are detectable by even a perfect electrical test.
Accordingly, it is an object of the invention to detect missing components on a PCB while the PCB is being electrically tested on an in-circuit tester.
It is also an object of the invention to detect missing components without physically probing the circuit.
It is yet another object of the invention to detect missing components that currently may not be detected by any prior art electrical test method in a manufacturing environment.